1. Field of the Invention
The present invention relates a polymer dispersed liquid crystal display device and, more particularly, to a polymer dispersed liquid crystal display device having a high response speed and capable of displaying an image having a high contrast.
2. Description of the Related Art
As a liquid crystal display device, a polymer dispersed liquid crystal display device has received a good deal of attention. A polymer dispersed liquid crystal display device is constituted by: a pair of substrates respectively having electrodes formed thereon; and a polymer dispersed liquid crystal film (a composite film consisting of a polymer resin and a liquid crystal) arranged between the pair of substrates and having a structure in which the polymer resin and the liquid crystal are dispersed.
As a liquid crystal constituting a polymer dispersed liquid crystal film, a nematic liquid crystal having positive dielectric anisotropy is used.
FIGS. 13A and 13B are enlarged sectional views respectively showing portions of the polymer dispersed liquid crystal film of a polymer dispersed liquid crystal display device. The polymer dispersed liquid crystal film shown in FIGS. 13A and 13B has a structure in which a nematic liquid crystal 102 is confined in spaces of a polymer resin 101 having a mesh-like sectional structure.
When no voltage is applied across the electrodes of both the substrates, an electric field is not applied to the polymer dispersed liquid crystal film. In this state, molecules LM of the nematic liquid crystal 102 in the polymer dispersed liquid crystal film have random directions, as shown in FIG. 13A.
In this state, the refractive indexes of the polymer resin 101 and the nematic liquid crystal 102 are different from each other, light incident on the liquid crystal display device is refracted and scattered at the interface between the polymer resin 101 and the nematic liquid crystal 102 and then scattered by the liquid crystal molecules LM having various directions. For this reason, a display becomes dark (turbid state).
When an electric field is applied across the electrodes of both the substrates, the molecules LM of the nematic liquid crystal 102 in the polymer dispersed liquid crystal film are uniformly aligned in the direction of the electric field as shown in FIG. 13B, i.e., in the direction almost perpendicular to the major surfaces of both the substrates. For this reason, the refractive index of the nematic liquid crystal 102 becomes almost equal to the refractive index of the polymer resin 101. Therefore, incident light is transmitted through the polymer dispersed liquid crystal film while almost no incident light is scattered, resulting in a bright display.
As described above, in the polymer dispersed liquid crystal display device, scattering and transmission of light in the polymer dispersed liquid crystal film are controlled by controlling a voltage applied across the electrodes, thereby displaying an image.
This polymer dispersed liquid crystal display device can display an image (bright/dark image) without using a polarizing plate. Therefore, unlike a generally used TN type liquid crystal display device or the like, an amount of light is not lost by optical absorption of a polarizing plate. Therefore, a screen brighter than that of a TN type liquid crystal display device can be obtained.
However, a conventional polymer dispersed liquid crystal display device has a low response speed, and a display image disadvantageously has a low contrast.
First, in consideration of the problem of a low response speed, when an electric field is applied, the liquid crystal molecules LM of the nematic liquid crystal 102 used in the polymer dispersed liquid crystal film are aligned in the direction of the electric field within a relatively short time in accordance with the strength of the applied electric field. However, when the application of the electric field is stopped, and the absence of an electric field is set, the direction of the liquid crystal molecules LM cannot be controlled by an electric field. For this reason, the state of the liquid crystal molecules LM is naturally returned to randomly aligned state. Therefore, it takes a long time to change the state of the display from a bright state to a dark state.
In this manner, a conventional polymer dispersed liquid crystal display device has a high response speed at which the state of the display is changed from the dark state to the bright state but a low response speed at which the state of the display is changed from the bright state to the dark state. Therefore, for example, a multiplex-driven liquid crystal display device cannot be easily time-sharing-driven at a high duty ratio.
In consideration of the problem of the low contrast of the display image, the molecules LM of the nematic liquid crystal 102 tend to be aligned along the inner surface of the polymer resin 101 in the absence of an electric field. For this reason, even when the molecules LM have random directions, the liquid crystal molecules LM are partially aligned in almost the same direction near the interface between the polymer resin 101 and the nematic liquid crystal 102 as shown in FIG. 13A.
For this reason, in a conventional polymer dispersed liquid crystal display device, the difference between the refractive indexes of the nematic liquid crystal 102 and the polymer resin 101 is small in the absence of an electric field. Therefore, the scattering efficiency of light is low, and the darkness of a dark display is insufficient, resulting in a display having a low contrast.